Mechanism of the interfacial reaction between cation-deficient La_0.56Li_0.33TiO₃ and metallic lithium at room temperature

We used x-ray diffractometry (XRD), x-ray photoelectron spectrometry (XPS), and secondary-ion mass spectrometry (SIMS) to investigate the mechanism of the interfacial room-temperature (RT) chemical reaction between cation-deficient La_0.56Li_0.33TiO₃ solid electrolytes and metallic lithium anodes in all-solid-state lithium batteries. A stoichiometric mixture of La₂O₃, Li₂CO₃, and TiO₂ powders was calcined at 1250 °C for 8 h to obtain a single perovskite structure of La_0.56Li_0.33TiO₃. When this La_0.56Li_0.33TiO₃ sample and lithium were placed in contact at room temperature for 24 h, the phase of the La_0.56Li_0.33TiO₃ remained unchanged. The XPS results indicate that 12% of the tetravalent Ti⁴⁺ ions were converted into trivalent Ti³⁺ ions. The valence conversion and degree of conversion were limited by the structural rigidity of the host crystal. Our SIMS analysis suggests the existence of a local electric field near the contact surface and indicates that the ⁶Li⁺ isotope ions were inserted into the specimen through the effect of this field. The change in the electrical properties of La_0.56Li_0.33TiO₃ supports this mechanism for the interfacial reaction. The ionic conductivities of the grain and total grain boundary decreased and increased, respectively, after the insertion of Li⁺, and the total electronic conductivity increased as a result of the presence of intervalence electron hopping between mixed Ti³⁺/Ti⁴⁺ states. The mechanism of the lithium-activated RT interfacial reaction is associated with the reduction of Ti⁴⁺ transition metal ions from tetravalent to trivalent states and the local-electric-field-induced Li⁺ insertion into La³⁺/Li⁺-site vacancies of La_0.56Li_0.33TiO₃.